Although the above is almost always true, there are a few exceptions...the new Leica cinema lenses, which are $150,000+ per set, are apparently as sharp as they get around t2 or t2.8. No one's going to mount that on a dSLR, but their new Summicron (which is $8000, I think--so that's a little more reasonable, not that I could ever afford it) is f2 and apparently sharpest wide open--pretty incredible, and if their cinema lenses are that sharp, maybe they can match that performance in a still lens.
The sharpest (wide) lens I've used wide open is the 35mm f1.4 Samyang, which is extremely inexpensive but it flares a bit and has some mild CA and, like all other lenses at reasonable prices, isn't at its theoretical sharpest until an f4/f5.6 split (though the center is sharp by f2.8 almost). It's useless to me for stills, though, due to its lack of autofocus. The 35mm L is sharper in the center but softer toward the edges.
That said, even at 18''X12'', you'll never notice the difference induced by either spherical aberration (wide stops) or diffraction (stopped down) so long as you're shooting normally. Technique is thousands of times more important at any normal stop. The sharpest prints I've ever seen were shot on 4x5 film, often at f64, which limits theoretical megapixels for that format to like less than 20? And the wall-sized (40''x50'' prints) were absolutely tack sharp. So I wouldn't worry unless you're printing wall-sized photos. A 100% crop represents an 80'' wide print on the 5D II or III and I can't tell the difference between f2.8 and f8 on a decent lens at 100%, even if software specifically designed to can.
If you must have sharp wide open, get a Leica M9 and the new Summicron, if you can afford the $15,000 investment. But it seems silly when a D4 or 1DX will have significantly superior high ISO performance to negate the difference in practice. What body do you have? Maybe a 1DX or D4 or other low light monster is a better investment?
